It is known that many receptor-ligand interactions are involved in the induction, establishment and modulation of immune responses directed against antigens. At least two signals are necessary to activate a CD4 or CD8 T-cell response to antigen (Lenschow et al., 1996). The first signal is delivered through the T-cell receptor (TCR) by an antigen (typically a peptide) bound to a major histocompatibility (MHC) class I or II molecule present on the surface of an antigen presenting cell (APC). The second signal involves the binding of a ligand present on the surface of the APC to a second receptor molecule on the surface of the T-cell. This second signal is termed co-stimulation, and the APC ligand is often referred to as a co-stimulatory molecule. The best characterized second signal is delivered via an interaction between the CD28 receptor on the T-cell, and its ligands B7.1 or B7.2 on the APC, although a number of other examples of receptor/co-stimulatory molecule interactions have been described.
In combination, the two signals activate the T-cell, which in turn secretes cytokines and proliferates. In the case of CD4 T-cells, the activated cells (designated CD4.sup.+) produce cytokines, including IL-2 and IFN.gamma., which activate killer (CD8.sup.+) T-cells at the site of inflammation. Once CD4 T-cells are activated, another receptor, CTLA-4 is expressed, which is homologous to CD28 and binds B7 molecules with a higher affinity than CD28. The B7/CTLA-4 interaction inhibits the activation signal of CD28 and delivers a negative signal that may down-regulate T-cell responses (Krummel et al., 1996; Walunas et al., 1996). This down-regulation mechanism may serve to prevent excessive immune system responses, for example by decreasing the amount of cytokines produced during an inflammatory event. Concurrently, however, it may also down-regulate the number of T-cells that go on to become "memory cells". Reducing the number of memory cells means that fewer such cells will be available to respond to the same antigen the next time it is encountered. However, there are a number of situations where it would be advantageous to maintain, rather than down-regulate, an active T-cell response. Cancer patients, for example, would benefit from maintaining an active T-cell response against tumor cells. The concept of vaccination requires that a population of memory T-cells which recognized the administered antigen be maintained.
Another receptor/ligand combination that has been proposed to play a role in co-stimulation of CD4 T-cells is the OX-40 receptor/OX-40 ligand pairing. While the CD28 receptor is present on the surface of many sub-classes of T-celis (irrespective of whether they are activated or not), the OX-40 receptor ("OX-40") (Paterson et al. 1987; Calderhead et al., 1993) has been shown to be present only on antigen activated CD4.sup.+ T-cells in vivo (Weinberg et al., 1994; 1996). Thus, it has been shown that OX-40 is present on activated CD4.sup.+ T-cells that recognize autoantigen at the site of inflammation in autoimmune disease, but not in the peripheral blood system (Weinberg et al, 1994; 1996). OX-40 has also been shown to be present on the surface a percentage of CD4.sup.+ T-cells isolated from tumor infiltrating lymphocytes and draining lymph node cells removed from patients with squamous cell tumors of the head and neck and melanomas (Vetto et al., 1997). The OX-40 ligand, a member of the tumor necrosis factor (TNF) superfamily, has been shown to co-stimulate T-cells which have been activated with an anti-CD-3 antibody (i.e., in a nonantigen-specific manner) (Godfrey et al., 1994). Beyond its general co-stimulatory function however, the biological role of the OX-40 receptor/OX-40 ligand interaction in the immune response pathway is, to date, unknown.